Rewrite system-text-json-net11 skill as imperative, decisive guidance (…

…#926)

The dotnet11/system-text-json-net11 skill scored 0%% pass across all five model families (issue #902, ADD-DECISIVENESS): it read as reference prose and the judge saw no behavior change vs baseline (7 ties, 67%% invocation).

Rewrite it into imperative when-A-do-B-verify-C guidance:
- Sharper frontmatter USE FOR / DO NOT USE FOR with trigger keywords to improve discovery.
- Decision table mapping each request to the exact API and the anti-pattern to avoid.
- Explicit DO/DON'T that target the eval's failure modes: no custom JsonNamingPolicy subclass, no cast of non-generic JsonTypeInfo, no try/catch to probe metadata.
- net11.0 run instructions (file-based app + project) so the program is actually executed.
- Verification checklist and common-pitfalls table.

Validated with skill-validator check (all checks passed).

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

fix mstest sdk eval timeout (#558)

* Fix timeout in MSTest.Sdk v3-to-v4 eval scenario 9

The fixture csproj had MSTest.Sdk/3.8.0 while the prompt claimed the user
already upgraded to 4.1.0. This mismatch caused the model to edit the
csproj and run dotnet build, triggering slow MSTest SDK NuGet downloads
that exceeded the 240s timeout on CI.

- Update fixture csproj to MSTest.Sdk/4.1.0 to match the prompt
- Add specific CS error codes to the prompt so the model provides direct
  fixes instead of building
- Update rubric item to reflect the project already being on 4.1.0

* Fix CI timeout in migrate-mstest-v3-to-v4 eval scenario

Reduce scope of the 'Fix multiple v4 breaking changes' scenario:
- Remove 3 lower-signal breaking changes from the fixture (AreEqual params,
  IsInstanceOfType out param, Properties.Contains) to cut agent work
- Add max_turns: 5 to cap agent iterations
- Lower timeout from 240 to 180 seconds

* Strengthen net6.0 scenario prompt for reliable skill activation